Such a quadrature phase shifter is already known in the art, e.g. from the European Patent Application EP-A1-0608577 of BELL TELEPHONE MANUFACTURING COMPANY N.V. entitled "GMSK modulator with automatic calibration" and published on Aug. 3, 1994. Therein, the first and second filter circuits provide respective first and second filter output signals which are roughly phase shifted by +45 degrees and -45 degrees with respect to a signal applied to the input. These roughly orthogonal filter output signals are amplified in their respective branches by the first and second amplifiers thereof. Each of the above first/second device used in this known phase shifter is a first/second multiplier which multiplies the first/second filter output signal of the first/second filter circuit with the phase adjust signal so as to provide a first/second device smaller output signal, i.e. a signal whose phase is the same as that of the first/second filter output signal but having a smaller amplitude. The device smaller output signal is then vectorially added to the larger amplified first/second filter output signal of the opposite branch by the second/first summing circuit and the results of these additions are the above second and first orthogonal output signals. An accurate angle of 90 degrees between these two output signals can thus be obtained by modifying the amplitude and/or the sign of the device smaller output signals. This modification is performed by changing the phase adjust signal which controls the devices, i.e. the multipliers. This phase adjust signal is for instance supplied by an external control circuit which measures the phase shift between the first and the second output signals.
This known phase shifter is used in the GSM (Global System for Mobile communications) and other radio transmitters and receivers where an accurate 90 degrees phase shift is necessary to obtain a sufficient image rejection in the transmitter and to have a correct 90 degrees phase difference between the I (In-phase) and Q (Quadrature-phase) base band phase vector signals in the receiver.
Over the past few years, Silicon Bipolar and GaAs MMIC's (Microwave Monolithic Integrated Circuits) were competing for the wireless phone radio transceiver application. As the circuits thereof required a 5 Volt supply, a known Gilbert cell could be perfectly used as a multiplier. Such a Gilbert cell is for instance known from the book "Analysis and design of analog integrated circuits" of P. R. Gray and R. G. Meyer, published by J. Wiley & Sons, New York, 1977, and more particularly from pages 563 to 575 thereof. It has, between the ground and the VCC (5 Volt) supply terminal, three levels of transistors: a bias signal or current source input level, a radio signal input level and a local oscillator input level.
Today, designers are struggling for the next step: to use a 3 Volt supply in battery operated wireless phones. Such a 3 Volt supply, with a tolerance of .+-.10%, allows to reduce the number of batteries, the cost, the volume and the weight of the handset. However, the above three transistor levels Gilbert cell can then no longer be used since the 1 Volt collector-to-emitter voltage drop (VCE) produced over each transistor, i.e. 3 Volts in total, leaves no headroom for the useful signal, especially when the supply voltage drops to 2.7 Volt at the end of the battery cycle. New circuits need thus to be designed.
Another problem with the above known phase shifter is the relative large amplitude difference of the different signals applied to the summing circuits, i.e. of the device smaller output signals and the larger signals at the output of the amplifiers. Indeed, in the transistor technique, e.g. from Chapter 4-6: "Frequency response of a transistor stage The short circuit current gain" at pages 121 to 126 of the McGraw-Hill book "Pulse, Digital and Switching Waveforms" by Millman and Taub (1965), it is well known that the transition frequency `fT` at which the short-circuit common-emitter current gain of a transistor attains unity and which thus corresponds to the bandwidth of the amplifier using the transistor, dramatically varies as a function of the emitter-to-collector voltage and/or the emitter current of this transistor. Because of the above large amplitude differences of the signals supplied to the first and second amplifiers and to the amplifiers included in the Gilbert cells, it is obvious that the gain thereof is not easy to control. As a result, the tuning of the known phase shifter is relatively difficult. This is especially true at the high frequencies at which it is operated in a radio transmitter or receiver.